Enhanced As(III) sequestration using sulfide-modified nanoscale

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Enhanced As(III) sequestration using sulfide-modified nanoscale zerovalent iron with a characteristic core-shell structure: Sulfidation and As distribution Deli Wu, Shuhan Peng, Kaili Yan, Binbin Shao, Yong Feng, and Yalei Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02787 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Sustainable Chemistry & Engineering

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Enhanced As(III) sequestration using sulfide-modified

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nanoscalezerovalent iron with a characteristic core-shell

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structure:Sulfidationand As distribution

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Deli Wu,*,a,bShuhanPeng,a,bKailiYan,a,bBinbinShao,a,bYong Feng,cYaleiZhang,a,b

5 6 7 8 9 10

a

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ABSTRACT:Sulfide-modified nanoscalezerovalent iron (S-nZVI) was synthesized and

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employed for the removal of aqueous As(III).The structure and removal performance of

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S-nZVIwas investigated and comparedwiththat of pristine nZVI. S-nZVI has an optimal As

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removal capacity of 240 mg/g, which is much higher than that ofnZVI.The sulfidation of

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nZVIalso enhanced the As(III) removal rate and the enhancement largely depended on the S/Fe

16

molar ratio.The optimum pH for As(III) removal with S-nZVIwas in a broad range from 3 to 8.

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Transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS),

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scanning electron microscopy (SEM) with EDS, and X-ray photoelectron spectroscopy

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(XPS)were employed to characterize the S-nZVI before and after reacting with As(III).The

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results demonstrated thatS-nZVI had a unique core-shell structure.Sulfur was incorporated into

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the shell of S-nZVI and the thickness of the surface layerincreased from 5 nm to approximately

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30 nm, which suggested that more As(III) could be sequestered by the nanoparticles.Therefore

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abetterAs(III) removal was therefore observed withas the S/Fe molar ratio increased.The As

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distribution in solid phasewas used to describe the As(III) removal mechanism,and the results

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revealed thatAs(III) removal is a complex process that includes surface adsorption and

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co-precipitation.Sulfidation

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outer-spherecomplexation, whichled to the enhanced As removal.Oxygen impaired the structure

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ofS-nZVI and generated oxidantsviairon sulfide transformations, whichdrove theAs(III)

29

oxidation, and contributed to the total As removal.The largeAs(III) removal capability and

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chemical stability of S-nZVIshow its potential as an effective and environmentally friendly

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material for As(III) removal from aqueous solutions.

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science &

Engineering, Tongji University, Shanghai, 200092, P. R. China b c

Shanghai Institute of Pollution Control and Ecological Security, Shanghai200092, P.R. China

Department of Civil Engineering, The University of Hong Kong,Pokfulam Road, Hong Kong, P.R. China

*Corresponding author. E-mail address: [email protected] (D. Wu).

promoted

the

precipitation

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ACS Paragon Plus Environment

process

and

inhibited

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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KEYWORDS: Sulfide-modified nZVI, Arsenite, Core-shell structure, Precipitation, Water

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remediation

35 36

Introduction

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Arsenic (As) is an environmental concern due to its toxicity and ubiquity in the environment.

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Thus various environmental protection agencies and national standards are lowering the

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permissible level of Asin drinking water to less than 10 µg·L-1.Water contamination by Asis a

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serious and ubiquitous global problem,especially in some developing regions, and there are

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reports aboutserious health problems in Bangladesh1, China2,caused by As-contaminated

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water.As is naturally present in different oxidation states, but the predominant forms of As in

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aqueous environments are arsenite (As(III)) and arsenate (As(V))3, 4.Compared with that of As

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(V), As (III) has a higher toxicity, mobility and solubility, and thus, it has attracted more attention

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5, 6

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Because of its small size, large surface effect, reducing properties and strong adsorption

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properties, nanoscalezerovalent iron (nZVI) has been widely used for wastewater treatment7-11.

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Although nZVI has advantages, such as the convenience of insitu application and low production

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of solid waste

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nZVI is limited, and the maximum adsorption of As(III)is lower than 30 mg/g 8, 13. Even surface

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modified-nZVIonly has a slightly enhancedremoval capacity a little14, 15. The other challenge is

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the poor chemical stability of nZVI, because thecations and anions in groundwater can affect its

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removal performance16, 17.

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The interaction between nZVI and heavy metals proceed via physical and chemical reactions,

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and due to the two functional constituents of nZVI, multiple reactive pathways occur. The main

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interaction mechanism is an adsorption, reduction and oxidation process18, 19. A growing body of

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evidence suggests that the oxide shell plays an important role in the treatmentof heavy metals20,

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21

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nZVIsurface. These metal ions are then adsorbed onto to the surface via electrostatic

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interactions/ surfacecomplexation and further immobilized by precipitation and reduction. The

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interactions of arsenate with nZVI are dominated by the formation of strong inner-sphere

. Therefore, As(III) was chosen as the target pollutant in this study.

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,nZVIalso has some drawbacks.For instance, the As removal capacity of pure

. The dissolved heavy metal ions first diffuse through the liquid membrane and reach the

2


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complexes on the nZVIsurface. Previous studies showed that the removal of As by nZVIis

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mainly related to the adsorption and co-precipitation processes on the nZVIsurface layer.Thus,

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optimizing the shell material may increase the activity of nZVI.

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Sulfidation is a recent and emerging method to stabilize iron nanoparticles. The use of

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acombination of Fe(0) and FeS might be a feasible strategy to produce a synergistic effect not

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available from individual components. Compared with nZVI, sulfurized nZVI (S-nZVI) has a

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higher activity for the removal of some heavy metals. Kim et al. 22, 23developed a new method to

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prepare nanoparticles by adding a sodium borohydride and dithionite mixture to ferric

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solutions,and they obtained a type of S-incorporated nZVI. Su et al.24used this type of S-nZVI

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nanoparticle for the adsorptive removal of cadmium (Cd). They discovered that the dithionite

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doping amount played a crucial role in the removal reactivity of the S-nZVI composite for

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Cd.The S-nZVI composite with a proper Fe/S ratio exhibited a larger adsorption capacity than

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that of pristine nZVI, and the effects of pH and aging can be ignored.Fan25, 26investigated an

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alternative strategy for the remediation of Tc-contaminated groundwater and discovered thatthe

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sequestration ofTc sulfide was favored under the sulfidic conditions stimulated by nZVI,which

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further confirm the importance of FeS in Tc sequestration. Therefore, sulfur plays a key role in

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the removal of pollutants, but the specific mechanism is not clear and should be further

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investigated.

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Currently, few studies have been reported on the removal of heavy metals using S-nZVI.S-nZVI

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has not yet been studied for As removal, and the reactivity of S-nZVIfor As removal has not been

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reported. As often binds with sulfurand the literature has reported that sulfur-bearing minerals

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such as pyrite and mackinawite, have good As removal performances27,

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al.29comparedthe removal of As(III) and As(V) in aqueous solutions (pH 5.5- 6.5) using goethite,

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lepidocrocite, mackinawite, and pyrite, and they demonstrated that mackinawite was more

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effective than the other iron-oxide phases. Under different pH conditions, the Fe and S in FeS

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combine with

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arsenopyrite (FeAsS), and thioarsenite(AsS33-)28, 30. These metal sulfides31, 32 are very stable, as

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indicated by their low solubility product constants,which means that the immobilized metals are

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unlikely to be re-released into water.Therefore,S-nZVI is likely to have a high removal capacity

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for As.

28

. Farquhar et

As to form different compounds, such as realgar (AsS), orpiment (As2S3),

3



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This study investigated the feasibility of using S-nZVI for As(III) removal from aqueous

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solutions. Because dithionite plays an important role in the synthesis of S-nZVI, the effects of

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dithionite on the structure of the synthesized S-nZVI and the removal of As(III) were examined.

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To comprehensively evaluate the S-nZVIcapacity, the potential influences of dissolved oxygen,

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pH, and anions on the removal of As(III) were studied. To explore the role of sulfurization, the

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reactivity of S-nZVI was compared with that of nZVI. To determine the removal mechanism for

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As, the composition, structure and morphology of pristine S-nZVI and used S-nZVI were

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characterized and compared.

100 101

Material and methods

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Materials

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All chemicals used in this study were of analytical grade and used as received. Ferrous sulfate

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(FeSO4), sodium borohydride (NaBH4), dithionite (Na2S2O4), sodium hydroxide(NaOH), sulfuric

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acid(H2SO4), sodium bicarbonate(NaHCO3),sodium dihydrogen phosphate(NaHPO4), Sodium

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benzoate (BA), p-HBA, tert-Butanol (TBA) and dimethyl sulfoxide (DMSO)were purchased

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fromSinopharmChemical Reagent Co., Ltd., China. Stock solutions of 1 g/L As(III)

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wereprepared from NaAsO2 (Sigma-Aldrich, purity 98%).Superoxide dismutase (SOD) was

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obtained from ShanghaiKayon Biological Technology Co., Ltd.Nanopure water (18.2 MU cm,

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Thermo) wasused to prepare all experimental solutions.

111 112

Preparation of nZVI and S-nZVI

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nZVI was prepared viathe reduction of Fe2+ ions with sodium borohydrideas a reducing agent

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according to the method described by Sun33 and Kim22. Briefly, 100 mL of a NaBH4 solution

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(0.4mol/L) was added dropwise into 100 mL of a FeSO4⋅ 7H2O (0.1 mol/L) solution under N2

116

protection in a three-neck flask. Then, the black nZVI products were separated and rinsed three

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times with deoxygenated water to remove impurities. The overall process occured according to

118

Eq(1).

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Fe2+ + 2BH4- + 6H2O → Fe0 + 2B(OH)3 + 7H2↑

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S-nZVI was synthesized using the method described by Kim et al. Briefly, 100 mLof 0.1M

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NaBH4 with different amounts ofNa2S2O4 (S/Fe ratios = 0.07, 0.14, 0.21, 0.28, 0.35) were

(1)

4


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separately introduced into a 100 mL FeSO4 solution (0.1M) via titration (titration rate

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~10mL/min).

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After the reduction, the nanoparticles were collected and washed with nanopure water three

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times. The freshly preparednZVI and S-nZVI were stored in ethanol at 4℃to prevent the

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materials from oxidization. The particles were dried in a vacuum oven for 1 d prior to their use

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and characterizations.

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Characterization of S-nZVI

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The surface morphology and composition were characterized by scanning electron microscopy

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coupled with an energy-dispersive X-ray (SEM/EDS) system at 15 kV (Hitachi S-4800)

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andtransmission electron microscopy (TEM, JEOL JEM 2011, Japan). The crystal phase of the

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particles was identified by X-ray diffraction (XRD, Bruker D8 Advance, Germany) using a Cu

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Ka (k = 1.54178 Å) radiation source, and the operation voltage and current were 40 kV and 40

135

mA, respectively. The diffraction angle (2h) was recorded from 10° to 80° with a scanning speed

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of 1°/min and a step size of 0.02°.X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA

137

System) was used to investigate the surface composition. A Zetasizer Nano-ZS90 instrument

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(Malvern, UK) was used to measure the zeta (ζ) potential of the particles.

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After reacting with the As(III) solutions, the aqueous samples were centrifuged for 5 min at 3500

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rpm, and the solid sample was freeze dried. The mineral composition of the adsorbents and the

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chemical state of the atoms present on the nanoparticle surface were investigated using XRD and

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XPS. The samples were analyzed in the C 1s, O 1s, S 2p, As 3d, and Fe 2p regions, which

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accounted for the major elements present on the surface. The morphological analysis of the

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pristine and used adsorbent was performed by SEM and TEM.

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As(III) removal performance of S-nZVI particles with different S/Fe ratios

147

S-nZVI (10mg/L) with different S/Fe ratios was accurately weighed and separately added to

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As(III) solutions (100 mL). Their pH values were then adjusted to 7 usingNaOH and H2SO4

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solutions,and no attempt was made to maintain a constant pH during the reaction. To simulate

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oxygen limited conditions, batch experiments were conducted in 100-mL headspace vials with

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Teflon-lined caps wrapped with aluminum foil. In addition,the As(III) solutions (5, 10, 20, 30, 5



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and 50mg/L) werepurged with nitrogen gas for 30 min prior to the addition of S-nZVI and during

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the reaction to exclude oxygen. The oxiccondition were achieved by exposing the samples to air.

154 155

As(III) removal with S-nZVIunder various conditions

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The As(III) removal efficiency of S-nZVI was compared that ofnZVI under a variety of

157

conditions.Certain amounts of S-nZVI were added to six As(III) solutions (10 mg/L, 100 mL) at

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pH values of 3, 5, 6, 7, 8, and 9. Afterreaction for 2 h, 1 mLof the supernatant was collected for

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the As, Fe, and S analyses via ICP-AES.

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The effects of anions (HPO42− and HCO3−) were studied using an initial As(III) concentration of

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10mg⋅L-1. Batch tests were performed using As(III) solutions containing 0.05, 0.1 and0.3mM of

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HPO42-or HCO3-. After reacting for 2 h, aliquots were collected from the supernatant and

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analyzed for As and Fe.

164 165

Analytical methods

166

Quantification of cumulative hydroxyl radicals (·OH)

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Certain amounts of S-nZVIand nZVIwere added to 100 mLexcessive BA solutions (2 g/L) at pH

168

= 7. The quenching experiments with additions of 2mMTBA, and 60 U/mL SOD were carried

169

out inthe same reactor.The reaction rate constantfor BA and ·OH is 5.7 × 109 M−1s−134, which is

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nearly diffusion-controlled.Theconversion of BA to p-HBA was used as a probe reaction.A

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conversionfactor of 5.87 was used to estimate the cumulative ·OHconcentrations35. The

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concentration of p-HBA was measured usingHPLC (Agilent 1260) equipped with a DAD

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detector andan Inter Sustain C18 column (4.6 × 250 mm). The mobilephase was a mixture of

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methanol and 0.1% glacial acetic acid aqueoussolution (30:70, v :v) at a flow rate of 1 mL/min,

175

with the detection wavelength at 255 nm.

176 177

As distribution on the solid phase

178

We used a sequential chemical extraction (SCE) procedure according to Handley et al. and

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Wenzel et al. 36, 37to investigate As distribution on the solid phases. After the reaction with As(III),

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the suspension was separated into an aqueous phase and solid phase by centrifugation at 3500

181

rpm for 5 min.The aqueous solid was collected, filtered through a 0.25-µm membrane filter, and 6


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182

acidified for analysis (residue: soluble in the initial aqueous phase).Then, the solid phase of the

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suspension was sequentially extracted using various extraction solutions: 0.5 M (NH4)2SO4for 4

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h (20 ℃)(exchangeable), 0.5 M NH4H2PO4for 16 h (specifically adsorption and surface-bound),

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0.2 M ammonium oxalate buffer (pH = 3.0) for 4 h in the dark(amorphous, hydrous iron

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oxide-associated),and 0.2 M ammonium oxalate buffer (pH = 3.0) and 0.1 M ascorbic acid for 30

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min (in the light) at 90℃ (well-crystallized hydrous iron oxide-associated). After each extraction

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step, the suspension was centrifuged at 3500 rpm for 5 min. The extracted As was separated from

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the supernatant, and the residue was retained for the next extraction step.

190 191

Results and discussion

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Characterization of S-nZVI

193

The S-nZVI was characterized by TEM and compared with the original nZVI (Fig.1). Based

194

onprevious studies, nZVIis generally recognized ashaving a core-shell structure18-20, 38.The TEM

195

images of S-nZVI show that the S-nZVIsurface has an obvious flake-like structures on the shell

196

with their diametersof approximately 200 nm, whichare larger than those of the flake-like

197

structures on nZVI (< 100 nm). The TEM with EDS characterization of S-nZVI(Fig.S1) revealed

198

that S/Feratio is0.26 in the flake-like region, and S/Fe ratio is0.1 in the nuclear region, indicating

199

that the flake-like material mainlycontained iron sulfidesand that sulfur was likelypresent in the

200

nucleus. SEM was performed to evaluate the changes in the surface topography caused by the

201

presence of Na2S2O4 during the nZVI synthesis process. When the S/Fe ratio was increased from

202

0 to 0.35, a significant increase in the surface roughness of the nanoparticles was observed,

203

which can be attributed to the formation of FeS precipitates. (b) )

(a) )

204 205

Fig.1TEM images of (a) nZVI and (b) S-nZVI particles with S/Fe ratio of 0.21.

206

The XRD patterns (Fig. S2) of nZVI and S-nZVI indicated that the main component of 7



Page 8 of 24

207

S-nZVIwasFe0.Only a small iron oxide peak was detected in S-nZVI, and a clear S0 or iron

208

sulfide peak was not observed. Because the surface of iron sulfide is amorphous instead of a

209

spinel structure, the iron sulfide on the S-nZVIsurfacecould not be detected by XRD.

210

To further verify the composition of S-nZVI, S-nZVI was characterized by XPS to compare the

211

surface composition of bare nZVIand S-nZVI, and to reveal the distribution of S and Fe along

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the particle depth.The results (Fig.2a) showed that the several S compositions propablypresent on

213

the S-nZVI surface, S2-(161.1eV), S22-(162.0eV), Sn2-(163.2eV)39,

214

SO42-(168.6 and 169.8eV). The Fe 2p spectra display strong Fe(II)-O and Fe(III)-O peaks in both

215

materials (Fig. 2e and Fig. S3). A relatively small peak was observed at 707.3 eV18

216

corresponding to Fe(0). The low signal was attributed to the fact that the X-ray probing depth

217

was less than 5 nm.

218

To determine if iron sulfide was predominantly present on the S-nZVI surface or uniformly

219

present throughout the particle, we conducted a depth profilevia XPS. The XPS spectra of Fe2p

220

and S 2pfor S-nZVI with and without etching are shown in Fig.2. A significant difference in the

221

spectra of Fe 2p at different etching depths was not observed,and peaks were present at 711.10

222

eV and 725.10 eV, which indicated that most of Fe on the S-nZVIwas oxidized. However,as the

223

depth increased, the proportion of Fe species increased, and the molar ratio of S/Fe decreased

224

from 0.23 to 0.14. The Fe 2p XPS spectra of nZVI showedFe(0) was on the surface layer of

225

nZVI(Fig. S4), which indicated that the shell thickness is very thin (less than 5 nm). Therefore,

226

comparingtheFe 2p XPS spectra for nZVI and S-nZVI shows that the thickness of the S-nZVI

227

shell is approximately 30nm.As the etching depth increased from 0 to 30 nm, the S species were

228

observed tobe homogeneously distributed through the surface layer of S-nZVI and half of the S

229

was present as S2- (the specific distribution of S species is shown in Fig. S5).Therefore, the shell

230

material of S-nZVI is speculated tobe composed of iron oxide and iron sulfide with a shell

231

thickness of approximately 30 nm, which is completely different from that of nZVI.

8


40

, SO32-(167.8eV) and

Page 9 of 24

2400

2800

(a)

(c)

2600

2-

Without etching

2200

S 2-

S2

2400

2000

(e) Etching depth=20nm

SO4

1800

2-

SO3

1600

Etching depth=60nm

S2-

S22-

2-

Sn

2-

CPS

CPS

2200 2000

Sn2-

1800 1600

1400

Etching depth=30nm

1200 172

170

168

166

164

162

1200 170

160

168

Binding Energy(eV)

166

164

162

160

158

Binding Energy(eV) 3500

3000 (b)

Etching depth=5nm

S2-

3000

(d) Etching depth=30nm

Sn

CPS

S222-

Etching depth=20nm

S22-

2500

2000

S2-

intensity

1400

CPS

Sn2-

2500

Etching depth=5nm

2000

Without etching

1500

1500

166

165

164

163

162

161

160

159

166

165

Binding Energy(eV)

164

163

162

161

160

740

Binding Energy(eV)

730

720

710

Binding Energy(eV)

232 233

Fig.2 The XPS spectra of (a-d) S2p and (e) Fe2p of the S-nZVI particles with different etching

234

depths.

235 236

S-nZVIAs removal performance

237

The effect of S/Fe ratios on As removal (a)

10

residual total As in solution (mg/L)

S/Fe = 0 S/Fe = 0.07 S/Fe = 0.14 S/Fe = 0.21 S/Fe = 0.28

8

6

4

2

0

238

30

60

90

120

S/Fe ratio 0 0.07 0.14 0.21 0.28

8

6

4

2

0 0

(b)

10

S/Fe ratio residual total As in solution (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

time (min)

60

120

180

240

time (min)

239

Fig.3As removal performance of S-nZVI with different S/Fe ratios under (a) oxic conditionsand

240

(b) nearly anoxic conditions

241

(initial As(III) = 10 mg/L; nanoparticle concentration = 100 mg/L; initial pH=7.0±0.1).

9


700


(a)

(b)

(c)

)

)

)

(d)

(e)

(f)

)

)

)

Page 10 of 24

242

243 244

Fig.4SEM images of (a) nZVI and S-nZVI particles with S/Fe ratio of (b) 0.07, (c) 0.14, (d) 0.21,

245

(e) 0.28, and (f) 0.35.

246

Table.1The elementatomic ratiosinnanoparticles with different S/Fe ratios At% Element S/Fe=0

S/Fe=0.07

S/Fe=0.14

S/Fe=0.21

S/Fe=0.28

S/Fe=0.35

O

22.24

15.7

10.83

20.20

33.68

30.22

S

3.41

6.52

7.88

8.96

9.69

10.96

Fe

74.35

77.77

81.30

70.85

56.65

58.82

0.045

0.07

0.10

0.13

0.17

0.18

measured S/Fe ratio 247

S-nZVI was modified by adding different concentrations of Na2S2O4during the nZVIsynthesis

248

process, and five different S/Fe molar ratios were obtained(S/Fe=0.07, 014, 0.21, 0.28, and 0.35).

249

The S/Fe ratio strongly influencedS-nZVIAs(III) removal performance. As the S/Fe ratio initially

250

increased, the As removal efficiency increased from 60% to 98% (Fig. 3). However, when the

251

S/Fe molar ratio increased to 0.28, the removal efficiency no longer increased, and the removal

252

rate decreased. The higher S/Fe ratio can slow down the reaction rate of S-nZVI, so the S/Fe

253

ratio has an optimal value.According to the SEM-EDScharacterization (Fig. 4 and Table. 1), the

254

S content increased as the S/Fe ratio increased in the S-nZVI. Hence, with an increasein the S

255

content, more iron sulfide formed, and the surface of the nanomaterials became rougher and

256

more irregular. The difference in the As(III) removal performance ofS-nZVIatdifferent S/Fe

257

ratios can be attributed to the different sulfidation extent. Several studies have suggested that 10


Page 11 of 24

258

surface modification of iron, such as pit formation and roughness,can cause dramatic

259

improvements in the removal efficiency41, 42, which could explain the improvement in the sulfide

260

As removal. Based on the SEM image, the morphology of the S-nZVI (S/Fe = 0.07) was closest

261

to that of the original nZVI, and the As(III)-removal capability of that material was close to that

262

of nZVI. In addition,As often binds with sulfur in nature, and thus theincrease in the sulfur

263

content on the surface is speculated to create more binding sites for As.

264

Fig.3 (a) and (b) show thatthe As removal of the S-nZVI waslargely influenced by the dissolved

265

oxygen content, and the efficiency of the As removal and reaction rate improved under aerobic

266

conditions. The other important difference between the two conditions is the total Fe

267

concentration trend in the supernatant (Fig. S6). Under aerobic conditions, the pH of the reaction

268

system increased from 7 to 8 and then decreased to approximately 6.5, whereas the pH increased

269

from approximately 7 to approximately 8 under limited oxygen conditions. Therefore, S-nZVI

270

surface has different reactions under aerobic and anaerobic conditions, and reactions (2) - (5)

271

occur under aerobic conditions. The Fe0 surface oxidized to Fe2+ and Fe3+and simultaneously

272

produced OH-.Therefore, the solution pH increased, and the hydrolysis of Fe3+via H+ decreased

273

the solution pH. Under anaerobic conditions, Fe0 slowly corrodes in water to produce OH-. S 2p

(a)

18000 2-

Sn 17500

2-

2-

SO4

SO3

0

S

2-

S2 CPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17000

2-

S

16500

16000 172

274

170

168

166

164

162

160

Binding Energy(eV)

11


158


2800

S 2p

(b)

2600 2400 2-

SO4

CPS

2200

2-

Fe7S8

2000

S

1800 1600 1400 172

170

168

166

164

162

160

Binding Energy(eV)

275 6000

5000

(c)

As 3d

(d)

As 3d

As(V) 5000

4000

As(V) As(III)

CPS

As(III)

CPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

4000

3000

3000

2000 50

48

46

44

42

50

48

Binding energy (eV)

276

46

44

42

Binding energy (eV)

277

Fig.5The XPS spectra of (a)b)S2pand (c) (d) As3dof the S-nZVIafter reaction

278

underoxic and anoxic conditions.

279

The increase in theAs removal capacity can be explained by the XPS analysis (Fig. 5). The S2p

280

region wasdeconvoluted into peaks for S2-(161.2eV), Fe7S8(162.8eV), Sn2-(163.2, 165eV),S0(164,

281

164.4eV),SO32- (167.8eV), and SO42- (168.6 and 170.0eV). The S2p spectra indicated the

282

existence of S0 and SO42-. Under oxic conditions, the dissolved oxygen contributed to breaking

283

the interior Fe-S bonds39, which converted them to sulfhydryl groups for As adsorption and led to

284

an increase in the As removal capacity.

285

As seen in the XPS spectra of As3d (Fig. 5(c) and (d)), most of the As(III) is oxidized to As(V)

286

under aerobic conditions, and it is only partially oxidized to As(V) under anaerobic

287

conditions.The proportion of As (V) in the reaction product of S-nZVI under oxic condition is

288

obviously higher than that of nZVI (Fig. S7).And many iron-based materials and the iron

289

corrosion productshave a higher removal efficiency for As(V)43. Previous findings have shown

290

that

291

FeSundercircumneutral conditions44.Meanwhile, the oxygenation of Fe(II) can also stimulate the

hydroxyl

radicals

(·OH)

can

be

efficiently

produced

12


viaoxygenation

of

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


292

formation of hydroxyl radicals, which generally conforms to the Haber-Weiss mechanism45, as

293

shown in reactions (9) - (13), and this can promote the oxidative transformation of

294

As(III)46-48.While the work conducted by Sedlak49 provides new evidence against ·OH as a

295

crucial Fenton intermediate at circumneutralpH. They found that at circumneutral and alkaline

296

pH, a more selective oxidant ferrylion (Fe(IV)) can be produced for the oxidation of As(III)

297

(reaction (14)).

298

In order to reevaluate the relative importance of the oxidants ·OH and Fe(IV) and their role in

299

As(III) oxidation, using DMSO(a Fe(IV) scavenger) ) as an indicator to distinguish

300

between ·OHand Fe(IV)50.It was found that the effects of DMSO on the As removal efficiency of

301

the S-nZVI and oxidation of As(III) were negligible (Fig. S8). ThereforeFe(IV) contributed to

302

negligibly to the oxidation of As(III).

303

Using thehydroxylation of BA to p-HBA as a probe reaction,46 thecumulative concentrations

304

of ·OH produced under aerobic with different dosagesS-nZVIand 0.5g/L nZVIwere

305

quantified.Fig. 6(a) shows that ·OH production of S-nZVIwas observed apparently increased

306

with the increasein S-nZVI dosage. And the production is much higher than that of nZVI at the

307

same dosage 0.5g/L. Therefore, sulfidation couldpromotenanoparticles to activate molecular

308

oxygen to produce·OHundercircumneutral conditions.To examine whether S-nZVI can activate

309

molecular oxygen to produce hydroxyl radicalsunder aerobic condition, the effect of TBA, an

310

•OH scavenger, was investigated.51.The result show that TBA with a concentration of 2

311

Mcompletely inhibited the production of ·OH (Fig. 6(b)). Similarly, under anaerobic

312

conditions, ·OH was also completely inhibited.In order to probe if S-nZVIthe number of

313

electrons transferred fromFe(II) to O2 for H2O2 production, the formation of O2·-, aone-electron

314

transfer intermediate, was examined. With theaddition of 60 U/mL SOD, an O2·- scavenger52,Fig.

315

6(b) shows that negligibleinfluence on ·OH production was observed. As a result, a two-electron

316

transfer process was supposed to predominate for the reduction of O2 to H2O2 during S-nZVI

317

oxygenation.Thus, theexposure of As(III) on S-nZVI resulted in the oxidation of As(III) by FeS

318

and an adsorption transformation by iron oxide.

319

4Fe0 + 3O2→ 6H2O + 4Fe(OH)3

320

2Fe0 + O2 + 2H2O → 2Fe2+ + 4OH-

321

4Fe2+ + O2 + 2H2O → 4Fe3+ + 4OH- (4)

(2) (3)

13



322

Fe3+ + 3H2O →Fe(OH)3 + 3H+ (5)

323

Fe0 + 2H2O → Fe2+ +H2 +2OH- (6)

324

FeS + 3/4O2 + 1/2H2O → 1/8S8 + γ-FeOOH (7)

325

FeS + 1/2O2 + 2H+→ Fe2+ + S0 + H2O

326

FeS + 2O2→ Fe2+ + SO42-(9)

327

Fe0 + 2H+ + O2→ Fe2+ + H2O2(10)

328

Fe2+ + H2O2→ Fe3+ + ·OH+ OH-(11)

329

Fe2+ + O2→ Fe3+ + O2·-(12)

330

Fe2+ + O2·-+ 2H+→ Fe3+ + H2O2(13)

331

Fe(II) + H2O2 →Fe(IV) + H2O

(8)

(14)

45

(a)

cumulative ·OH produced (µM)

40 35 30

0.1 g/L S-nZVI 0.2 g/L S-nZVI 0.5 g/L S-nZVI 0.5 g/L nZVI

25 20 15 10 5 0 -5 0

60

120

180

240

300

time (min)

332 45

(b)

40 cumulative ·OH produced (µM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

35 30 25 20

0.5 g/L S-nZVI 0.5 g/L S-nZVI anoxic 0.5 g/L S-nZVI with 2 M TBA 0.5 g/L S-nZVI with 60 U/ml SOD

15 10 5 0 -5 0

333

60

120

180

240

300

time (min)

334 335

Fig. 6 (a) Cumulative·OH productionat different dosagesof S-nZVI. (b) Effects of oxygen, TBA and SOD on ·OH production at0.5 g/L S-nZVI (initial pH = 7).

336

The influence of oxygen on S-nZVIAs removal was also analyzed via SEM and XRD (Fig. S9,

337

S10). Under limited oxygen conditions, the structure of S-nZVI did not change significantly.The

338

flake-like structure and the majority of the core remained. However, in the present of oxygen, the

339

structure of S-nZVI remarkably changed, and flake-like deposits with a needle-like framework 14


Page 15 of 24

340

were observed.

341 342

The effect of pH on As removal

343

It was found that pH influencedboth dissolution and As removal of S-nZVI(Fig. 7, Fig. S11).At

344

pH = 3, 5, and 9,the reaction rate and removal efficiency wereobviouslyinhibited.While when the

345

pH wasin the range of 6-8, it is less affected by pH.This was consistent with the change in the pH

346

during the reaction process(Fig. S11).TheAs removal kinetics can be divided into two stages,

347

S-nZVI corrosion (eq. (2), (3), and (4)) and oxidation (eq. (5)). Fig. 7 and Fig. S11showed that

348

S-nZVIAs(III) removal performance for pH 3.0 - 9.0was mainly limited by the S-nZVI corrosion

349

rate.

350

However the As(III) removal rate was still superior to that of nZVI.One possible reason for this

351

is thatnear-complete dissolution of nZVI occurred when pH < 4 after 2 h of reaction, whereas

352

only a small part of S-nZVI dissolved under identical conditions. The isoelectric points (IEPs) of

353

nZVI and S-nZVIare approximately pH = 7.5 and5.5 respectively (Fig. S12). The IEP of S-nZVI

354

was lower than nZVI, indicating that FeS(IEP pH = 0.8 - 3.5) on the surfaceinfluenced the

355

surface charge. ThereforeS-nZVIis negatively charged in most natural water systems.At 0

Enhanced As(III) sequestration using sulfide-modified nanoscale

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